Conventionally, addition of various fillers to resins has been studied for imparting characteristics of the fillers to the resins. For example, addition of a glass fiber, a carbon fiber, or the like has been studied for imparting rigidity, and addition of a filler of metal such as copper or aluminum, or a carbon filler such as graphite, carbon black, or carbon nanotube has been studied for imparting electrical conductivity (for example, Japanese Unexamined Patent Application Publication No. 2007-5547 (PTL 1), Japanese Unexamined Patent Application Publication No. 2010-155993 (PTL 2)).
In particular, olefin-based resins are excellent in terms of the balance between the price and the mechanical properties, and are one of the most widely used types of resin. Various fillers such as graphite particles have been conventionally added to such olefin-based resins for imparting various characteristics. For example, Japanese Unexamined Patent Application Publication No. Sho 59-96142 (PTL 3) discloses that an electrically conductive filler such as carbon black or graphite is incorporated into a thermoplastic resin such as polypropylene, so that electrical conductivity is imparted to the thermoplastic resin, and the electromagnetic wave shielding property of the thermoplastic resin is improved.
However, when a glass fiber, a carbon fiber, a metal filler, or a carbon filler is simply mixed, as it is, with a resin as described in the PTLs 1 to 3, it cannot be said that the characteristics of the filler are sufficiently imparted to the resin. For example, graphite particles are easily aggregated, have a low affinity for resins, and hence are dispersed in a resin in an aggregated state. The aggregated graphite particles make it difficult to sufficiently exhibit their characteristics such as heat resistance, chemical resistance, mechanical strength, thermal conductivity, and electrical conductivity. In particular, graphite particles have a remarkably low affinity for olefin-based resins such as polypropylene, and are easily aggregated and difficult to uniformly disperse in an olefin-based resin. Hence, it cannot be said that electrical conductivity and mechanical properties of the graphite particles are sufficiently imparted to the olefin-based resin.
In this respect, various methods have been proposed for highly dispersing graphite particles in a resin. For example, disclosed are a method in which a surface of a carbon filler such as graphite is modified with a carboxylic acid ester to change the properties of the surface, and then the modified carbon filler is added to a polymer (for example, International Application Japanese-Phase Publication No. 2002-508422 (PTL 4)), a method in which graphite oxide having an organic onium ion intercalated therein is melt kneaded with a thermoplastic resin (Japanese Unexamined Patent Application Publication No. 2006-233017 (PTL 5)), and the like. Moreover, although not directed to graphite particles, a method in which a nano carbon composite material whose surface is coated with a polyimide-based resin or the like is added (Japanese Unexamined Patent Application Publication No. 2006-144201 (PTL 6)), a method in which a lamellar carbon subjected to a hydrogenation or alkylation treatment is uniformly and finely dispersed in a resin (Japanese Unexamined Patent Application Publication No. 2003-268245 (PTL 7)), and the like are disclosed as methods for highly dispersing a nano carbon.
However, when a carbon material is subjected to a surface modification treatment as described above, the characteristics (in particular, electrical conductivity) of the carbon material tend to be impaired. For this reason, although the carbon material is highly dispersed in the resin, the characteristics are not imparted sufficiently. Hence, the methods described in PTL 4 to 7 still have room for improvement. In particular, there is a problem that even when an alkylation treatment is conducted on graphite particles by using an alkylzinc compound according to the method described in PTL 7, surfaces of the graphite particles are not alkylated, and further the conjugated structure on the surfaces of the graphite particles is destructed, and the electrical conductivity is lowered. Moreover, since organometallic compounds such as alkylzinc compounds are unstable, and difficult to handle, this method is unsuitable for industrial production.